There is no file. Each MLB stadium has a folder in Z:\.......\OOTP Baseball 26\data\ballparks\models
There are tutorials on how to install downloaded parks in the OOTP Mods - 3D Models forum.
https://forums.ootpdevelopments.com/...lay.php?f=3879
YouTube has tutorials on how assign ballparks.
https://www.youtube.com/results?sear...ign+a+ballpark